Electric heaters are used extensively in connection with plastic molding machines, one type of heater being the band type of heater which surrounds or extends at least part way around a part. Another type of electric heater is the cartridge type of heater adapted to be inserted within a part. The cartridge type has been used in so-called "runnerless" molding machines in which a plastic is injected directly into a mold cavity from the tip of a bushing which is heated to an elevated temperature to permit plastic flow therethrough and which is referred to as a "hot tip bushing." With such an arrangement, it is possible to eliminate sprues and runners and to reduce cycling time. In the bushing, the plastic flows from a receiving end to an exit end through passages which are on the outside of a central torpedo shaped element which encloses the cartridge heater. The cartridge heater, which includes a coiled heating element, may also include a thermocouple junction at the end thereof closest to the exit end of the bushing. The provision of the thermocouple junction makes a heater more expensive to produce but it is advantageous in that the temperature may be sensed at a point close to the exit end of the bushing and the operation may be more accurately controlled to obtain optimum results.
Runnerless plastic molding machines have been proposed using a single passage bushing in which the plastic in flowing from the receiving end to the exit end flows in a single central passage rather than through a plurality of parallel passages. This arrangement has a number of advantages including the fact that any one of a number of important types of plastics can be used which could not be used if the plastic were to be separated and then reunited as is the case with the first-described type of hot tip bushing having parallel passages. With the single passage bushing, use of a band type of heater and a separate thermocouple is indicated. The cartridge type of heater is unsuitable for the single passage bushing, except that a plurality of such heaters are disposed in parallel relation to and around a passage as disclosed in the Crandell U.S. Pat. No. 3,970,821.
The Halliday U.S. Pat. No. 3,849,630 discloses a heating device in which a length of mineral insulated heating cable extends along a helical passage between a core and a sleeve, the core being formed with a helical groove in its outer surface and the sleeve being formed with a mating helical groove in its inner surface. The cable sheath is clamped between the core and the sleeve. This arrangement may be used in a plastic molding bushing and has certain potential advantages with respect to heat transfer but the provision of the grooves would make it difficult and expensive to manufacture. Also, there is no provision for sensing of temperature or for the control of heat.